She died on unknown death because

She dwelt among the untrodden ways    
Beside the springs of Dove,    
A Maid whom there were none to praise    
And very few to love :    
A violet by a mossy stone    
Half hidden from the eye!    
—Fair as a star, when only one    
Is shining in the sky.    
She lived unknown, and few could know    
When Lucy ceased to be;    
But she is in grave, and, oh,    
The difference to me!    
— William Wordsworth